1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, more particularly to a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer, with the transparency thereof being reversibly changeable depending upon the temperature thereof, which is capable of recording information therein and erasing recording information therefrom repeatedly as desired by utilizing the reversibly changeable transparency of the reversible thermosensitive recording layer.
2. Discussion of Background
Recently, reversible thermosensitive recording media, which are capable of temporarily forming images or recording information therein and also capable of erasing formed images or recorded information therefrom when such formed images or recorded information becomes unnecessary, have attracted attention.
For instance, Japanese Laid-Open Patent Application 55-154198 discloses such a reversible thermosensitive recording medium provided with a thermosensitive recording layer which comprises an organic low-molecular weight material such as a higher fatty acid, which is dispersed in a matrix resin such as a vinyl chloride-vinyl acetate copolymer and whose transparency reversibly changes depending upon the temperature thereof.
Such a reversible thermosensitive recording medium, however, has the shortcomings that formed images are erased under high temperatures. This is because a higher alcohol or higher fatty acid with a melting point in the range of 60.degree. to 80.degree. C. is employed as the organic low-molecular-weight material, so that the reversible thermosensitive recording layer becomes transparent at temperatures in the range of 60.degree. to 80.degree. C.
Therefore, when such a reversible thermosensitive recording medium bears recorded images thereon and is placed at a hot place, for instance, on a dashboard of a car, which is exposed to the summer sun light, the recorded images are erased. Therefore, such a reversible thermosensitive recording medium is not suitable for use in cars, for instance, for the material for a prepaid card for toll expressway.
In order to eliminate this shortcoming of the conventional reversible thermosensitive recording medium by improving the heat resistance of formed images, a method of using a ketone with a higher alkyl group or a semicarbazone as the organic low-molecular-weight material has been proposed in Japanese Laid-Open Patent Application 3-230993. This method has made it possible to shift the transparency temperature at which the reversible thermosensitive recording layer becomes transparent to a higher temperature, so that the shortcoming that recorded images are completely erased at high temperatures has been eliminated. However, this method still has the shortcoming that the milky white degree of the reversible thermosensitive recording layer is low and accordingly the contrast of recorded images is also low.
Furthermore, in order to improve the durability of the reversible thermosensitive recording medium during repeated image formation and erasure thereof, a method of using a matrix resin having a glass transition temperature (Tg) of 80.degree. C. or more has been proposed in Japanese Laid-Open Patent Application 4-110187.
This method is capable of solving the problem of the lowering of image contrast during the repeated use of the reversible thermosensitive recording medium to some extent, but cannot solve the problem satisfactorily.